A basho_bench driver for load generating Riak CS, of second type.
First type driver (`basho_bench_driver_cs') is good at load
generating with large HTTP payload. It has measurament option that
offer data transfer throughput like MB/sec. On the other hand,
this driver's purpose is for small objects. If the first type is
used for small objects, user and bucket objects can be hot keys in
Riak. This one can handle multiple users and buckets to avoid it.